The present inventors previously proposed an emulsion polymerization process which comprises: (A) employing as a dispersant a water soluble carbonyl group-containing copolymer comprising (a) 0.5 to 99.5% by weight of a carbonyl group-containing monomer unit having at least one aldehyde or keto group and a polymerizable double bond per molecule; (b) 99.5 to 0.5% by weight of a monomer unit selected from among a monoolefinically unsaturated carboxylic acid having 3 to 5 carbon atoms, an amide of said unsaturated carboxylic acid, an N-alkyl and/or N-alkylol derivative of said unsaturated carboxylic acid amide and a monoolefinically unsaturated sulfonic acid; and (c) 0 to 70% by weight of a monomer unit selected from among an alkyl (meth)acrylate an alkyl moiety of which has 1 to 8 carbon atoms, a vinyl aromatic compound, a vinyl halide, ethylene, acrylonitrile, methacrylonitirle, a saturated carboxylic acid vinyl ester and a 1,3-diene, with the total amount of the monomer units as defined in (a) to (c) being 100% by weight and (B) emulsion-polymerizing, including copolymerizing, unsaturated monomer(s) having polymerizable double bond(s) (cf. Japanese Laid-Open Patent Application No. 104902/83). However, subsequent studies have revealed that a film of a polymer dispersion composition, which is prepared by adding a hydrazine derivative having at least two hydrazine residues per molecule to the polymer dispersion as obtained by the above process, is unsatisfactory in water-resistant anti-whitening, water-resistant adhesiveness and rust resistance, though said film is excellent in blocking resistance, solvent resistance and adhesiveness to an alkyd resin film.